1. Technical Field
This invention relates to the preparation of polysaccharide aldehydes using selective oxidation conditions. More particularly, this invention involves the preparation of polysaccharide aldehyde derivatives using nitroxyl radical mediated aqueous oxidation with a limited amount of oxidant and defined reaction conditions to provide derivatives with maximum effective aldehyde and minimal carboxylic acid content. This invention further involves the use of these selectively prepared polysaccharide aldehydes as strength additives in papermaking.
2. Background Information
The preparation of aldehyde containing polysaccharides and the use of such aldehyde derivatives as wet and dry strength additives in the paper industry is well known. Oxidative and non-oxidative methods have been used to introduce aldehyde groups into polysaccharides such as starches, gums and celluloses. Oxidative methods are well known for introducing aldehyde groups into polysaccharides such as starch. For example, U.S. Pat. No. 3,553,193 issued Jan. 5, 1971 to D. H. Leroy et al. teaches treatment of starch with alkali metal bromite or hypobromite under carefully controlled conditions; b) oxidizing a carbohydrate with an alkali metal ferrate is shown in U.S. Pat. No. 3,632,802 issued Jan. 4, 1972 to J. N. BeMiller; c) enzymatically oxidizing hydroxypropyl galactoglycoside starch ethers or ethyl galactoglycoside starch ethers with galactose oxidase is disclosed in U.S. Pat. No. 4,663,448 issued May 5, 1987 to C. W. Chiu; and d) treatment with periodic acid or periodates is disclosed in U.S. Pat. No. 3,096,969 issued Apr. 23, 1963 to J. E. Slager, which shows the preparation of dialdehyde polysaccharide using periodic acid, and U.S. Pat. No. 3,062,652 issued on Nov. 6, 1962 to R. A. Jeffreys et al. which shows the preparation of dialdehyde gums using periodate or periodic acid.
Oxidative methods often cause degradation of the polysaccharide and the formation of excess carboxyl groups. Excess carboxyl groups undesirably reduces the degree of aldehyde substitution and negatively affects the cationic/anionic balance of the functional groups when using a cationic polysaccharide base in papermaking applications.
The use of nitroxyl radicals and nitrosonium salts in organic chemistry as an oxidative route to access aldehydes and carboxylic acids from primary and secondary alcohols is disclosed in an article entitled Organic Nitrosonium Salts As Oxidants in Organic Chemistry by J. M. Bobbitt and C. L. Flores, in Heterocycles, Vol. 27, No. 2, pp. 509-533 (1988). Recently, application of this chemistry was extended to the selective oxidation of primary alcohols in various carbohydrates to carboxylic acids in an article entitled Selective Oxidation of Primary Alcohols Mediated by Nitroxyl Radical in Aqueous Solution. Kinetics and Mechanism by A. E. J. de Nooy and A. C. Bessemer, in Tetrahedron, Vol. 51, No. 29, pp. 8023-8032 (1995). Patent publication WO 95/07303 dated Mar. 16, 1995 further discloses the use of this technology where carbohydrates having a primary hydroxyl group are oxidized under aqueous conditions to form products having a high content of greater than 90% carboxyl groups. This art involving the oxidation of primary alcohols generally describes the preparation of polyglucuronic acids with high carboxylic acid content. Similarly, the process of oxidation has been used to prepare various polysaccharides with high carboxyl content as described in Oxidation of Primary Alcohol Groups of Naturally Occurring Polysaccharides with 2,2,6,6-Tetramethyl-1-piperidine Oxoammonium Ion by P. S. Chang and J. F. Robyt in J. Carbohydrate Chemistry, 15(7), pp. 819-830 (1996). It should be noted that in some applications high carboxylic content is undesirable.
A recent patent publication, WO 99/23240 dated May 14, 1999 discloses a method for producing oxidized starch using an oxoammonium ion-producing reagent in the presence of an enzyme-oxidizing agent.
Polysaccharide aldehydes may also be prepared by non-oxidative methods, which typically involve the reaction of a polysaccharide with an aldehyde-containing reagent. For example, U.S. Pat. No. 4,675,394 issued to D. Solarek et al. on Jun. 23, 1987 discloses a non-oxidative method wherein polysaccharide is reacted with a derivatizing acetal reagent in the presence of alkali to provide the stable acetal polysaccharide. The general disadvantage of these non-oxidative approaches is that they often require an extra conversion step to the aldehyde. For example, the polysaccharide acetal needs to be cooked or pre-treated under acidic conditions to form the aldehyde just prior to its use.
What is desired is a direct and selective oxidation route to polysaccharide aldehyde derivatives wherein the aldehydes are formed at maximum effective levels and carboxylic acid levels are minimized making such aldehyde derivatives especially useful as wet, temporary wet and dry strength additives for paper.